1. Field of the Invention
The present invention concerns a disk-shaped optical information recording medium and, more specifically, it relates to an optical information recording medium for recording main information, for example, users' information recorded in the form of pits, as well as sub-information, for example, administrative information in the form of bar-codes.
2. Description of the Related Art
In existent disk-shaped optical information recording medium (“optical disk”), administrative information such as serial numbers or lot numbers are bar-coded and recorded thereon. Then, the serial numbers or bar-codes are used for the method of distinguishing whether the medium is an optical information recording medium of authorized manufacturers or dealers.
For example, in an optical information recording medium such as DVD-ROM, a method of recording a bar-code like mark (hereinafter referred to as a “BCA mark”) in a BCA (Burst Cutting Area), and reading the BCA mark by an optical head provided to a playback device for reproducing the optical information recording medium, has been proposed and put to practical use.
JP-T-2005-518055 (the term “JP-T-” as used herein means a published Japanese translation of a PCT patent application) describes an optical information recording medium 100. An example of such a medium is shown in FIG. 4. The optical information recording medium 100 has an optical reflection layer, a phase change recording layer and a transparent layer of about 0.1 mm thickness on a disk-shaped substrate of about 120 mm outer diameter, about 15 mm inner diameter and about 1.1 mm thickness. The optical information recording medium 100 is provided with a BCA 101 having BCA marks 104 recorded in a range from about 21 mm to about 22 mm from the center of the substrate, a playback-only area 102 in a range from about 22.4 mm to about 23.2 mm from the center, and a recording and playback area 103 in a range from about 23.2 mm to about 58.6 mm from the center. Then, users' information can be recorded in the recording and playback area 103 by irradiating a laser beam at a wavelength of about 405 nm from an optical head with a number of aperture of about 0.85 from the side of the transparent layer of the optical information recording medium 100. The phase change recording layer is rewritable. For this purpose, it has been proposed to widen the track pitch in the BCA area 101 by about 5 times compared with the track pitch in the recording and playback area 103, thereby preventing tampering of the BCA mark 104 by the information recording drive when accessing the recording and playback area 103. For forming the BCA mark, a red laser at a wavelength of about 650 nm and with a high power of about 900 mW is used. Accordingly, at a portion where the laser beam is irradiated, the phase change recording layer and the optical reflection layer are burnt out by the laser beam to form an opening and the reflectivity at the point has a value approximate to 0%.
For reading the BCA mark, a laser beam at a wavelength of about 405 nm is irradiated by using an optical head with a number of aperture of about 0.85 from the side of the transparent layer of the optical information recording medium 100. The laser beam irradiated to the optical information recording medium 100 and reflected at the optical reflection layer is detected again by the optical head, and put to analog-digital (AD) conversion with a threshold value at 30% reflectivity and the absence or presence of the BCA mark is judged usually. Specifically, in a case where there is a difference of 30% or more between the reflectivity in a non-BCA mark portion and the reflectivity of the BCA mark portion in the BCA area, this is detected as the BCA mark by the playback device.
Further, JP-A-2006-85791 describes that a BCA area is disposed in the same manner as described above to an optical information recording medium having an organic dye recording layer of no reversible change instead of a rewritable phase change recording layer. Specifically, this is an HD DVD-R type optical information recording medium having an optical absorption layer of an organic dye material and an optical reflection layer on a transparent substrate of about 120 mm outer diameter, about 15 mm inner diameter, and about 0.6 mm thickness. Then, in the optical information recording medium, a BCA area is disposed within a range from about 22.2 mm to about 23.2 mm from the center of the substrate, and an administrative information area is disposed within a range from about 23.4 mm to about 23.8 mm from the center of the substrate. Further, a user's information area is disposed within a range from about 23.8 mm to about 58.5 mm from the center of the substrate. Then, user's information can be recorded to the user's information area by irradiating a laser beam at a wavelength of about 400 μm to about 420 μm on the side of a transparent substrate of the optical information recording medium. Further, the medium has grooves in which the track pitch in the BCA area having mirror portions and grooves put between the mirror portions is narrower than the track pitch in the user's information area. Then, by forming a BCA mark in the BCA area, BCA marks can be formed to aimed positions at an accuracy higher than the positional accuracy of the BCA mark recording apparatus.
The optical information recording medium described above with respect to JP-T-2005-518055 has an optical reflection layer, phase change recording layer, and an optical transparency layer of about 0.1 mm thickness on a substrate of about 1.1 mm thickness. Then, in the optical information recording medium, the track pitch in BCA area 101 is widened by about 5 times compared with the track pitch in the recording and playback area 103. In this optical information recording medium, use of a recording layer containing an organic dye instead of the phase change recording layer is to be examined. In the BCA area, increases in light interference lower the reflectivity in an area where grooves are formed. On the other hand, in a land area where the track pitch is widened by about 5 times, a high reflectivity can be obtained. Accordingly, the difference of the reflectivity between the land area and the area where the grooves are formed (hereinafter referred to as “reflectivity difference”) is extended to 30% or more. Accordingly, this involves a problem of causing mis-decipherment for the bar code information while erroneously taking the lowering of the reflectivity in the area where the grooves are formed for a portion of BCA marks upon reading the BCA marks.
Further, the optical information recording medium described above with respect to JP-A-2006-85791 has an optical absorption layer of an organic dye material and an optical reflection layer on a transparent substrate of about 0.6 mm thickness. Then, the optical information recording medium has grooves with the track pitch in the BCA area narrower than the track pitch in the users' information area. Then, in the optical information recording medium, the depth of the groove (D) is relatively large relative to the width (W) of the groove, measured approximately equidistant from the top and bottom of the groove, in the BCA area (hereinafter referred to as “half-band width”). Accordingly, increases in light interference in the area where the grooves are formed lower the reflectivity in the same manner as described above and the difference between the reflectivity of the land area and the reflectivity of the area where the grooves are formed (hereinafter referred to as “reflectivity difference”) is extended to 30% or more. Therefore, this leads to a problem that occurrence of mis-decipherment for the bar code information is inevitable.